Srimad-Bhagavatam: Canto 12 - Chapter 1 - Verse 42
Sanskrit:
तस्य ह्यासंस्त्रयो वर्णा अकाराद्या भृगूद्वह ।धार्यन्ते यैस्त्रयो भावा गुणनामार्थवृत्तय: ॥ ४२ ॥
ITRANS:
tasya hy āsaṁs trayo varṇāa-kārādyā bhṛgūdvahadhāryante yais trayo bhāvāguṇa-nāmārtha-vṛttayaḥ
Translation:
Oṁkāra exhibited the three original sounds of the alphabet — A, U and M. These three, O most eminent descendant of Bhṛgu, sustain all the different threefold aspects of material existence, including the three modes of nature, the names of the Ṛg, Yajur and Sāma Vedas, the goals known as the Bhūr, Bhuvar and Svar planetary systems, and the three functional platforms called waking consciousness, sleep and deep sleep.